Conventional hot air furnaces used in residential and commercial buildings generally include a fuel burner which burns oil, gas or other suitable fuel. The burner heats a heat exchanger which is positioned in a flow path of air to be heated (often termed “comfort air”). Heat from the combustion chamber is transferred to the air and the air is then distributed throughout the building.
Many prior art furnaces include rather large heat exchangers in order to maximize the heat extracted from the combustion of fuel. The more efficiency desired, the larger the heat exchanger must be. Alternately, supplementary heat exchangers must be provided.
In many prior art furnaces, the air necessary for the combustion of fuel is drawn from the immediate vicinity of the furnace. The by-products of combustion are then conveyed to a flue or chimney and are exhausted directly to the environment. In this type of furnace, the so-called “stack temperature” is high and as a result the exhaust products carry away a substantial quantity of heat and exhaust it to the chimney or flue. This heat loss substantially reduces the efficiency of the furnace.
The need for conserving energy has, in recent years, prompted the design of more efficient furnaces. Attempts at improving the efficiency of a hot air furnace have included designs intended to recover some of the heat lost “up the chimey”. One such design included the use of a supplementary heat exchanger that is heated by the exhaust by-products and which is also positioned in the path of air being heated. In one arrangement known as a condensing mode furnace, the supplementary heat exchanger is positioned at the comfort air inlet so that the air is preheated before passing over the primary heat exchange surfaces (i.e., the surfaces heated directly by the combustion chamber). As the comfort air passes through the supplementary heat exchanger, heat is absorbed from the heat exchanger and the temperature of the flue gases is substantially reduced. If the supplemental heat exchanger is properly designed, water vapor is condensed, hence the term “condensing mode” furnace. As is known, a change in state of water from vapor to liquid causes a substantial release of energy which is transferred to the comfort air.
A condensing mode furnace can substantially reduce the stack temperature of the combustion by-products. In many, if not most of these types of furnaces, a supplementary blower often termed an “induced draft” blower is used to encourage the flow of exhaust gases to the flue, chimney or other discharge conduit.
An example of a condensing mode furnace that utilizes an “induced draft” blower is described in U.S. Pat. No. 4,718,401, which is hereby incorporated by reference. Furnaces constructed in accordance with the '401 patent have proven commercially successful. However, it has been found that under certain operating conditions, some instability in the burning process may occur.
Additionally, it has also been observed in prior art furnaces that products of combustion condense prior to reaching the condenser. This reduces the efficiency of the furnace and leads to poor heating of comfort air. Premature condensation may also result in corrosion of furnace components upstream of the condensing heat exchanger.